A major equipment need among the applicant user group is a stable, sensitive, versatle EPR spectrometer capable of generating precise conventional and saturation-transfer EPR data in the absorption and the dispersion modes. The instrument should be interfaced with a computer to control its functions and to manipulate and store the data acquired. High sensitivity is necessary in order to obtain data with samples of low spin concentration. Stability is important for saturation transfer, to perform melting curves, and to collect data over long periods of time. The IBM-Bruker ER/200D EPR spectrometer is a modular system allowing purchase of only the components required for the planned experiments, yet retaining the flexibility of adding additional components at a later time when the long term objectives of the research may change or when new applications of EPR and related techniques may develop, e.g. Q band EPR, ENDOR, ELDOR. The IBM-Bruker ER/200D fulfulls the requirements for the varied projects planned. Although the Varian E109E will continue to be useful it is no longer adequate to carry out most of the experiments described in the application. The major projects for which the requested instrument will be used are concerned with the understanding of the molecular aspects of contraction of muscle and non-muscle systems and of the structure and function of intra- and extra-cellular membranes and the cytoskeleton. The various Principal Investigators are studying: 1) the molecular mechanisms underlying the movements of Ca2+ and protons across the membranes of the sarcoplasmic reticulum and mitochondria, respectively, intestinal absorption by microvilli, and the molecular basis of anesthesia; ii) the binding of the released Ca2+ to the thin filament and the resulting important conformational changes which allow contraction to take place; iii) the molecular mechanisms that are associated with the hydrolysis of ATP and muscle contraction; and iv) the function of F-actin in non-muscle cells. In order to study the relevant interactions within and between the protein constituents of these systems at different levels of purification and reconstitution various chemical and physical parameters are monitored. For all of these projects EPR spectroscopy has become a most useful technique.